The present invention relates to cryopumps of the kind which incorporate, within a thermally insulated enclosure, trapping means comprising refrigerated walls for trapping by condensation, and walls for trapping by adsorption which are covered with at least one layer of adsorbent substances such as activated carbon or zeolites. By combining these two trapping means it is possible, at least in theory, to achieve extremely high vacua, of the order of 10.sup.-x Torr, by virtue of the fact that gases which are difficult to condense even at very low trapping pressures, such as hydrogen and neon, are trapped by the walls for trapping by adsorption. However, the effectiveness of the walls for trapping by adsorption is considerably reduced by the fact that they become contaminated by more easily condensed gages such as nitrogen, oxgen and argon. It has proved possible to alleviate this drawback by placing the walls for trapping by adsorption downstream of the walls for trapping by condensation with reference to the flow of the gases, which latter walls also advantageously include a wall forming a baffle which shields the walls for trapping by adsorption from direct impingement of the gases.
A baffle of this kind has at least three disadvantages. Firstly, it is expensive to produce, secondly, if it is to be efficient under molecular conditions it needs to be optically sealed, which results in a considerable reduction in the pumping rate of the adsorbent covered surface, and finally, no matter what its design, it performs virtually no protective function for the adsorbent then the conditions under which the gases flow in the enclosure are viscous or intermediate, which may occur in certain applications of the cryopump.
Another method of pumping by adsorption has also been proposed which consists in depositing on the cryosurface a gas which condenses at the temperature concerned and which as the property of being adsorbent to the incondensable gases (a deposit of CO.sub.2 for example has the property of being adsorbent to hydrogen at 20.degree. K.). Firstly, this deposit of adsorbent gas suffers from the same drawbacks as a solid adsorbent, as described above, and secondly, when the adsorbent gas is injected onto the cryosurface there is always a rise in pressure in the enclosure either as a result of an excess of injected gas or as a result of surface heating of the cryodeposit already present, and this rise in pressure is generally undesirable.
A first object of the invention is to provide a sorption cryopump having a high pumping efficiency, i.e. a maximum pumping rate under molecular conditions and good protection for the adsorbent under viscous or intermediate conditions.
A second object of the invention is to provide a cryopump in which adsorbent gas is injected but any rise in pressure in the enclosure when the adsorbent gas is injected is prevented.